I just got the book ‘From After Effects to Flash’ and in it the author mentions an intermittent bug in AE7 related to your crash. He also says that Adobe is aware of it and will fix it in the next version of AE.
Finally, he says that you’re better of exporting a QT (in a codec that handles an alpha) and importing that into the Flash Video Encoder that came with Flash 8. Of course if you don’t have Flash 8 you’ll have a problem.
Good book.